User: added helper `as_json` method

This will be useful for bootstrapping models in the client and to avoid an
extra initial request.
User: moved model away from admin
User: added `gravatarUrl()` method
User: removed unnecessary XHR response parsing
User: adjusted comment
XHR API: removed the ability to return rendered forms

Clients should rather perform this on their own.
Webpack: decoupled output filepath from the admin dir
User: moved profile editing markup to the client

Fixes #270.